Memory utilizing oxide-nitride nanolaminates

ABSTRACT

Structures, systems and methods for transistors utilizing oxide-nitride nanolaminates are provided. One transistor embodiment includes a first source/drain region, a second source/drain region, and a channel region therebetween. A gate is separated from the channel region by a gate insulator. The gate insulator includes oxide-nitride nanolaminate layers to trap charge in potential wells formed by different electron affinities of the oxide-nitride nanolaminate layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/933,050, filed Sep. 2, 2004, which is a divisional of U.S.application Ser. No. 10/190,689, filed Jul. 8, 2002, both of which areincorporated herein by reference.

This application is related to the following co-pending, commonlyassigned U.S. patent applications: “Memory Utilizing OxideNanolaminates,” Ser. No. 10/190,717, and “Memory UtilizingOxide-Conductor Nanolaminates,” Ser. No. 10/191,336 each of whichdisclosure is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor integratedcircuits and, more particularly, to gate structures utilizingoxide-nitride nanolaminates.

BACKGROUND OF THE INVENTION

Many electronic products need various amounts of memory to storeinformation, e.g. data. One common type of high speed, low cost memoryincludes dynamic random access memory (DRAM) comprised of individualDRAM cells arranged in arrays. DRAM cells include an access transistor,e.g a metal oxide semiconducting field effect transistor (MOSFET),coupled to a capacitor cell.

Another type of high speed, low cost memory includes floating gatememory cells. A conventional horizontal floating gate transistorstructure includes a source region and a drain region separated by achannel region in a horizontal substrate. A floating gate is separatedby a thin tunnel gate oxide. The structure is programmed by storing acharge on the floating gate. A control gate is separated from thefloating gate by an intergate dielectric. A charge stored on thefloating gate effects the conductivity of the cell when a read voltagepotential is applied to the control gate. The state of cell can thus bedetermined by sensing a change in the device conductivity between theprogrammed and un-programmed states.

With successive generations of DRAM chips, an emphasis continues to beplaced on increasing array density and maximizing chip real estate whileminimizing the cost of manufacture. It is further desirable to increasearray density with little or no modification of the DRAM optimizedprocess flow.

Multilayer insulators have been previously employed in memory devices.The devices in the above references employed oxide-tungsten oxide-oxidelayers. Other previously described structures described have employedcharge-trapping layers implanted into graded layer insulator structures.

More recently oxide-nitride-oxide structures have been described forhigh density nonvolatile memories. All of these are variations on theoriginal MNOS memory structure described by Fairchild Semiconductor in1969 which was conceptually generalized to include trapping insulatorsin general for constructing memory arrays.

Studies of charge trapping in MNOS structures have also been conductedby White and others.

Some commercial and military applications utilized non-volatile MNOSmemories.

However, these structures did not gain widespread acceptance and use dueto their variability in characteristics and unpredictable chargetrapping phenomena. They all depended upon the trapping of charge atinterface states between the oxide and other insulator layers or poorlycharacterized charge trapping centers in the insulator layersthemselves. Since the layers were deposited by CVD, they are thick, havepoorly controlled thickness and large surface state charge-trappingcenter densities between the layers.

Thus, there is an ongoing need for improved DRAM technology compatibletransistor cells. It is desirable that such transistor cells befabricated on a DRAM chip with little or no modification of the DRAMprocess flow. It is further desirable that such transistor cells provideincreased density and high access and read speeds.

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BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) in a substrate according to the teachings of theprior art.

FIG. 1B illustrates the MOSFET of FIG. 1A operated in the forwarddirection showing some degree of device degradation due to electronsbeing trapped in the gate oxide near the drain region over gradual use.

FIG. 1C is a graph showing the square root of the current signal (Ids)taken at the drain region of the conventional MOSFET versus the voltagepotential (VGS) established between the gate and the source region.

FIG. 2A is a diagram of an embodiment for a programmed transistor,having oxide-nitride nanolaminate layers, which can be used as atransistor cell according to the teachings of the present invention.

FIG. 2B is a diagram suitable for explaining a method embodiment bywhich a transistor, having oxide-nitride nanolaminate layers, can beprogrammed to achieve the embodiments of the present invention.

FIG. 2C is a graph plotting the current signal (Ids) detected at thedrain region versus a voltage potential, or drain voltage, (VDS) set upbetween the drain region and the source region (Ids vs. VDS).

FIG. 3 illustrates a portion of an embodiment of a memory arrayaccording to the teachings of the present invention.

FIG. 4 illustrates an embodiment of an electrical equivalent circuit forthe portion of the memory array shown in FIG. 3.

FIG. 5 illustrates an energy band diagram for an embodiment of a gatestack according to the teachings of the present invention.

FIG. 6 is a graph which plots electron affinity versus the energybandgap for various insulators.

FIGS. 7A-7B illustrates an embodiment for the operation of a transistorcell having oxide-nitride nanolaminate layers according to the teachingsof the present invention.

FIG. 8 illustrates the operation of a conventional DRAM cell.

FIG. 9 illustrates an embodiment of a memory device according to theteachings of the present invention.

FIG. 10 is a schematic diagram illustrating a conventional NOR-NORprogrammable logic array.

FIG. 11 is a schematic diagram illustrating generally an architecture ofone embodiment of a programmable logic array (PLA) with logic cells,having oxide-nitride nanolaminate layers according to the teachings ofthe present invention.

FIG. 12 is a block diagram of an electrical system, or processor-basedsystem, utilizing oxide nanolaminates constructed in accordance with thepresent invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

The terms wafer and substrate used in the following description includeany structure having an exposed surface with which to form theintegrated circuit (IC) structure of the invention. The term substrateis understood to include semiconductor wafers. The term substrate isalso used to refer to semiconductor structures during processing, andmay include other layers that have been fabricated thereupon. Both waferand substrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator, aswell as other semiconductor structures well known to one skilled in theart. The term conductor is understood to include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

FIG. 1A is useful in illustrating the conventional operation of a MOSFETsuch as can be used in a DRAM array. FIG. 1A illustrates the normal hotelectron injection and degradation of devices operated in the forwarddirection. As is explained below, since the electrons are trapped nearthe drain they are not very effective in changing the devicecharacteristics.

FIG. 1A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) 101 in a substrate 100. The MOSFET 101 includes asource region 102, a drain region 104, a channel region 106 in thesubstrate 100 between the source region 102 and the drain region 104. Agate 108 is separated from the channel region 108 by a gate oxide 110. Asourceline 112 is coupled to the source region 102. A bitline 114 iscoupled to the drain region 104. A wordline 116 is coupled to the gate108.

In conventional operation, a drain to source voltage potential (Vds) isset up between the drain region 104 and the source region 102. A voltagepotential is then applied to the gate 108 via a wordline 116. Once thevoltage potential applied to the gate 108 surpasses the characteristicvoltage threshold (Vt) of the MOSFET a channel 106 forms in thesubstrate 100 between the drain region 104 and the source region 102.Formation of the channel 106 permits conduction between the drain region104 and the source region 102, and a current signal (Ids) can bedetected at the drain region 104.

In operation of the conventional MOSFET of FIG. 1A, some degree ofdevice degradation does gradually occur for MOSFETs operated in theforward direction by electrons 117 becoming trapped in the gate oxide110 near the drain region 104. This effect is illustrated in FIG. 1B.However, since the electrons 117 are trapped near the drain region 104they are not very effective in changing the MOSFET characteristics.

FIG. 1C illustrates this point. FIG. 1C is a graph showing the squareroot of the current signal (Ids) taken at the drain region versus thevoltage potential (VGS) established between the gate 108 and the sourceregion 102. The change in the slope of the plot of √{square root over(Ids)} versus VGS represents the change in the charge carrier mobilityin the channel 106.

In FIG. 1C, ΔVT represents the minimal change in the MOSFET's thresholdvoltage resulting from electrons gradually being trapped in the gateoxide 110 near the drain region 104, under normal operation, due todevice degradation. This results in a fixed trapped charge in the gateoxide 110 near the drain region 104. Slope 103 represents the chargecarrier mobility in the channel 106 for FIG. 1A having no electronstrapped in the gate oxide 110. Slope 105 represents the charge mobilityin the channel 106 for the conventional MOSFET of FIG. 1B havingelectrons 117 trapped in the gate oxide 110 near the drain region 104.As shown by a comparison of slope 103 and slope 105 in FIG. 1C, theelectrons 117 trapped in the gate oxide 110 near the drain region 104 ofthe conventional MOSFET do not significantly change the charge mobilityin the channel 106.

There are two components to the effects of stress and hot electroninjection. One component includes a threshold voltage shift due to thetrapped electrons and a second component includes mobility degradationdue to additional scattering of carrier electrons caused by this trappedcharge and additional surface states. When a conventional MOSFETdegrades, or is “stressed,” over operation in the forward direction,electrons do gradually get injected and become trapped in the gate oxidenear the drain. In this portion of the conventional MOSFET there isvirtually no channel underneath the gate oxide. Thus the trapped chargemodulates the threshold voltage and charge mobility only slightly.

One of the inventors, along with others, has previously describedprogrammable memory devices and functions based on the reverse stressingof MOSFET's in a conventional CMOS process and technology in order toform programmable address decode and correction in the U.S. Pat. No.6,521,950 entitled, “MOSFET Technology for Programmable Address Decodeand Correction.”. That disclosure, however, did not describe write onceread only memory solutions, but rather address decode and correctionissues. One of the inventors also describes write once read only memorycells employing charge trapping in gate insulators for conventionalMOSFETs and write once read only memory employing floating gates. Thesame are described in co-pending, commonly assigned U.S. patentapplications, entitled “Write Once Read Only Memory Employing ChargeTrapping in Insulators,” Ser. No. 10/177,077 and “Write Once Read OnlyMemory Employing Floating Gates,” Ser. No. 10/177,083. The presentapplication, however, describes transistor cells having oxide-nitridenanolaminate layers and used in integrated circuit device structures.

According to the teachings of the present invention, transistor cellscan be programmed by operation in the reverse direction and utilizingavalanche hot electron injection to trap electrons in oxide-nitridenanolaminate layers of the transistor. When the programmed transistor issubsequently operated in the forward direction the electrons trapped inthe oxide-nitride nanolaminate layers cause the channel to have adifferent threshold voltage. The novel programmed transistors of thepresent invention conduct significantly less current than conventionaltransistor cells which have not been programmed. These electrons willremain trapped in the oxide-nitride nanolaminate layers unless negativecontrol gate voltages are applied. The electrons will not be removedfrom the oxide-nitride nanolaminate layers when positive or zero controlgate voltages are applied. Erasure can be accomplished by applyingnegative gate voltages and/or increasing the temperature with negativegate bias applied to cause the trapped electrons in the oxide-nitridenanolaminate layers to be re-emitted back into the silicon channel ofthe transistor.

FIG. 2A is a diagram of an embodiment for a programmed transistor cell201 having oxide-nitride nanolaminate layers according to the teachingsof the present invention. As shown in FIG. 2A the transistor cell 201includes a transistor in a substrate 200 which has a first source/drainregion 202, a second source/drain region 204, and a channel region 206between the first and second source/drain regions, 202 and 204. In oneembodiment, the first source/drain region 202 includes a source region202 for the transistor cell 201 and the second source/drain region 204includes a drain region 204 for the transistor cell 201. FIG. 2A furtherillustrates the transistor cell 201 having oxide-nitride nanolaminatelayers 208 separated from the channel region 206 by a first oxide 210.An sourceline or array plate 212 is coupled to the first source/drainregion 202 and a transmission line 214 is coupled to the secondsource/drain region 204. In one embodiment, the transmission line 214includes a bit line 214. Further as shown in FIG. 2A, a gate 216 isseparated from the oxide-nitride nanolaminate layers 208 by a secondoxide 218.

As stated above, transistor cell 201 illustrates an embodiment of aprogrammed transistor. This programmed transistor has a charge 217trapped in potential wells in the oxide-nitride nanolaminate layers 208formed by the different electron affinities of the insulators 208, 210and 218. In one embodiment, the charge 217 trapped in the oxide-nitridenanolaminate layers 208 includes a trapped electron charge 217.

FIG. 2B is a diagram suitable for explaining the method by which theoxide-nitride nanolaminate layers 208 of the transistor cell 201 of thepresent invention can be programmed to achieve the embodiments of thepresent invention. As shown in FIG. 2B the method includes programmingthe transistor. Programming the transistor includes applying a firstvoltage potential V1 to a drain region 204 of the transistor and asecond voltage potential V2 to the source region 202.

In one embodiment, applying a first voltage potential V1 to the drainregion 204 of the transistor includes grounding the drain region 204 ofthe transistor as shown in FIG. 2B. In this embodiment, applying asecond voltage potential V2 to the source region 202 includes biasingthe array plate 212 to a voltage higher than VDD, as shown in FIG. 2B. Agate potential VGS is applied to the control gate 216 of the transistor.In one embodiment, the gate potential VGS includes a voltage potentialwhich is less than the second voltage potential V2, but which issufficient to establish conduction in the channel 206 of the transistorbetween the drain region 204 and the source region 202. As shown in FIG.2B, applying the first, second and gate potentials (V1, V2, and VGSrespectively) to the transistor creates a hot electron injection intothe oxide-nitride nanolaminate layers 208 of the transistor adjacent tothe source region 202. In other words, applying the first, second andgate potentials (V1, V2, and VGS respectively) provides enough energy tothe charge carriers, e.g. electrons, being conducted across the channel206 that, once the charge carriers are near the source region 202, anumber of the charge carriers get excited into the oxide-nitridenanolaminate layers 208 adjacent to the source region 202. Here thecharge carriers become trapped in potential wells in the oxide-nitridenanolaminate layers 208 formed by the different electron affinities ofthe insulators 208, 210 and 218.

In an alternative embodiment, applying a first voltage potential V1 tothe drain region 204 of the transistor includes biasing the drain region204 of the transistor to a voltage higher than VDD. In this embodiment,applying a second voltage potential V2 to the source region 202 includesgrounding the sourceline or array plate 212. A gate potential VGS isapplied to the control gate 216 of the transistor. In one embodiment,the gate potential VGS includes a voltage potential which is less thanthe first voltage potential V1, but which is sufficient to establishconduction in the channel 206 of the transistor between the drain region204 and the source region 202. Applying the first, second and gatepotentials (V1, V2, and VGS respectively) to the transistor creates ahot electron injection into the oxide-nitride nanolaminate layers 208 ofthe transistor adjacent to the drain region 204. In other words,applying the first, second and gate potentials (V1, V2, and VGSrespectively) provides enough energy to the charge carriers, e.g.electrons, being conducted across the channel 206 that, once the chargecarriers are near the drain region 204, a number of the charge carriersget excited into the oxide-nitride nanolaminate layers 208 adjacent tothe drain region 204. Here the charge carriers become trapped inpotential wells in the oxide-nitride nanolaminate layers 208 formed bythe different electron affinities of the insulators 208, 210 and 218, asshown in FIG. 2A.

In one embodiment of the present invention, the method is continued bysubsequently operating the transistor in the forward direction in itsprogrammed state during a read operation. Accordingly, the readoperation includes grounding the source region 202 and precharging thedrain region a fractional voltage of VDD. If the device is addressed bya wordline coupled to the gate, then its conductivity will be determinedby the presence or absence of stored charge in the oxide-nitridenanolaminate layers 208. That is, a gate potential can be applied to thegate 216 by a wordline 220 in an effort to form a conduction channelbetween the source and the drain regions as done with addressing andreading conventional DRAM cells.

However, now in its programmed state, the conduction channel 206 of thetransistor will have a higher voltage threshold and will not conduct.

FIG. 2C is a graph plotting a current signal (IDS) detected at thesecond source/drain region 204 versus a voltage potential, or drainvoltage, (VDS) set up between the second source/drain region 204 and thefirst source/drain region 202 (IDS vs. VDS). In one embodiment, VDSrepresents the voltage potential set up between the drain region 204 andthe source region 202. In FIG. 2C, the curve plotted as 205 representsthe conduction behavior of a conventional transistor where thetransistor is not programmed (is normal or not stressed) according tothe teachings of the present invention. The curve 207 represents theconduction behavior of the programmed transistor (stressed), describedabove in connection with FIG. 2A, according to the teachings of thepresent invention. As shown in FIG. 2C, for a particular drain voltage,VDS, the current signal (IDS2) detected at the second source/drainregion 204 for the programmed transistor (curve 207) is significantlylower than the current signal (IDS1) detected at the second source/drainregion 204 for the conventional transistor cell (curve 205) which is notprogrammed according to the teachings of the present invention. Again,this is attributed to the fact that the channel 206 in the programmedtransistor of the present invention has a different voltage threshold.

Some of these effects have recently been described for use in adifferent device structure, called an NROM, for flash memories. Thislatter work in Israel and Germany is based on employing charge trappingin a silicon nitride layer in a non-conventional flash memory devicestructure. Charge trapping in silicon nitride gate insulators was thebasic mechanism used in MNOS memory devices, charge trapping in aluminumoxide gates was the mechanism used in MIOS memory devices, and one ofthe present inventors, along with another, has previously disclosedcharge trapping at isolated point defects in gate insulators. However,none of the above described references addressed forming transistorcells utilizing charge trapping in potential wells in oxide-nitridenanolaminate layers formed by the different electron affinities of theinsulators.

FIG. 3 illustrates an embodiment for a portion of a memory array 300according to the teachings of the present invention. The memory in FIG.3, is shown illustrating a number of vertical pillars, or transistorcells, 301-1, 301-2, . . . , 301-N, formed according to the teachings ofthe present invention. As one of ordinary skill in the art willappreciate upon reading this disclosure, the number of vertical pillarare formed in rows and columns extending outwardly from a substrate 303.As shown in FIG. 3, the number of vertical pillars, 301-1, 301-2, . . ., 301-N, are separated by a number of trenches 340. According to theteachings of the present invention, the number of vertical pillars,301-1, 301-2, . . . , 301-N, serve as transistors including a firstsource/drain region, e.g. 302-1 and 302-2 respectively. The firstsource/drain region, 302-1 and 302-2, is coupled to a sourceline 304. Asshown in FIG. 3, the sourceline 304 is formed in a bottom of thetrenches 340 between rows of the vertical pillars, 301-1, 301-2, . . .., 301-N. According to the teachings of the present invention, thesourceline 304 is formed from a doped region implanted in the bottom ofthe trenches 340. A second source/drain region, e.g. 306-1 and 306-2respectively, is coupled to a bitline (not shown). A channel region 305is located between the first and the second source/drain regions.

As shown in FIG. 3, oxide-nitride nanolaminate layers, shown generallyas 309, are separated from the channel region 305 by a first oxide layer307 in the trenches 340 along rows of the vertical pillars, 301-1,301-2, . . . , 301-N. In the embodiment shown in FIG. 3, a wordline 313is formed across the number of pillars and in the trenches 340 betweenthe oxide-nitride nanolaminate layers 309. The wordline 313 is separatedfrom the pillars and the oxide-nitride nanolaminate layers 309 by asecond oxide layer 317.

FIG. 4 illustrates an electrical equivalent circuit 400 for the portionof the memory array shown in FIG. 3. As shown in FIG. 4, a number ofvertical transistor cells, 401-1, 401-2, . . . , 401-N, are provided.Each vertical transistor cell, 401-1, 401-2, . . . , 401-N, includes afirst source/drain region, e.g. 402-1 and 402-2, a second source/drainregion, e.g. 406-1 and 406-2, a channel region 405 between the first andthe second source/drain regions, and oxide-nitride nanolaminate layers,shown generally as 409, separated from the channel region by a firstoxide layer.

FIG. 4 further illustrates a number of bit lines, e.g. 411-1 and 411-2.According to the teachings of the present invention as shown in theembodiment of FIG. 4, a single bit line, e.g. 411-1 is coupled to thesecond source/drain regions, e.g. 406-1 and 406-2, for a pair oftransistor cells 401-1 and 401-2 since, as shown in FIG. 3, each pillarcontains two transistor cells. As shown in FIG. 4, the number of bitlines, 411-1 and 411-2, are coupled to the second source/drain regions,e.g. 406-1 and 406-2, along rows of the memory array. A number of wordlines, such as wordline 413 in FIG. 4, are coupled to a gate 412 of eachtransistor cell along columns of the memory array. According to theteachings of the present invention, a number of sourcelines, 415-1,415-2, . . . , 415-N, are formed in a bottom of the trenches betweenrows of the vertical pillars, described in connection with FIG. 3, suchthat first source/drain regions, e.g. 402-2 and 402-3, in columnadjacent transistor cells, e.g. 401-2 and 401-3, separated by a trench,share a common sourceline, e.g. 415-1. And additionally, the number ofsourcelines, 415-1, 415-2, . . . , 415-N, are shared by column adjacenttransistor cells, e.g. 401-2 and 401-3, separated by a trench, alongrows of the memory array 400. In this manner, by way of example and notby way of limitation referring to column adjacent transistor cells, e.g.401-2 and 401-3, separated by a trench, when one column adjacenttransistor cell, e.g. 401-2, is being read its complement columnadjacent transistor cell, e.g. 401-3, can operate as a reference cell.

FIG. 5 illustrates an energy band diagram for an embodiment of a gatestack according to the teachings of the present invention. As shown inFIG. 5, the embodiment consists of insulator stacks, 501-1, 501-2 and501-3, e.g. SiO₂/oxide-nitride nanolaminate layers/SiO₂. The first andthe last layer, 501-1 and 501-2, are done by atomic layer deposition.The disclosure describes the fabrication by atomic layer deposition,ALD, and use of oxide-nitride-oxide nanolaminates, as shown in FIG. 5,and consisting of the nitride materials, shown in FIG. 6, as follows:

(i) silicon nitride

(ii) aluminum nitride

(iii) gallium nitride

(iv) gallium aluminum nitride

(v) tantalum aluminum nitride

(vi) titanium silicon nitride

(vii) titanium aluminum nitride

(viii) tungsten aluminum nitride

In embodiments of the invention, these nitride material are of the orderof 4 nanometers in thickness, with a range of 1 to 10 nm. Thecompositions of these materials are adjusted so as that they have anelectron affinity less than silicon which is 4.1 eV, resulting in apositive conduction band offset as shown in FIG. 5. The materials can bedeposited by ALD as described in the next section.

FIG. 6 illustrates how the electron affinity varies with compositionratio of the compound materials. These results have been superimposedupon the graph of the values for the electron affinities of the commonmaterials, silicon. This difference in electron affinities for instancecreates the barrier between silicon and oxide of around 3.2 eV.Generally it is observed that as the bandgap of materials increases theelectron affinity decreases.

The electron affinity of GaN is the subject of numerous reports.

AlN is a low electron affinity material. For example, UV photoemissionmeasurements of the surface and interface properties of heteroepitaxialAlGaN on 6H—SiC grown by organometallic vapor phase epitaxy (OMVPE) showa low positive electron affinity for Al/sub 0.55/Ga/sub 0.45/N sampleand GaN, whereas the AlN samples exhibited the characteristics ofnegative electron affinity. On the other hand, in semi-insulating anddegenerate n-type GaN samples prepared by chemical vapor deposition withheat-cleaned surface the electron affinity was found to lie between 4.1and 2.1 eV.

Assuming the electron affinity of GaN to be around 2.7 eV, the electronaffinity decreases for GaAIN as the aluminum composition increases untilthe material becomes AlN which has a lower electron affinity of around0.6 eV as shown in FIG. 6.

Titanium nitride, tantalum nitride and tungsten nitride are mid-gap workfunction metallic conductors commonly described for use in CMOS devices.

Method of Formation

This disclosure describes the use of oxide-nitride nanolaminate layerswith charge trapping in potential wells formed by the different electronaffinities of the insulator layers. These layers formed by ALD are ofatomic dimensions, or nanolaminates, with precisely controlledinterfaces and layer thickness. Operation of the device specificallydepends on and utilizes the electron affinity of the nitride layer beinghigher than that of silicon oxide. This creates a potential energy wellin the multi-layer nanolaminate gate insulator structure.

Atomic Layer Deposition of Nitrides

Ta—N: Plasma-enhanced atomic layer deposition (PEALD) of tantalumnitride (Ta—N) thin films at a deposition temperature of 260° C. usinghydrogen radicals as a reducing agent forTertbutylimidotris(diethylamido) tantalum have been described. The PEALDyields superior Ta—N films with an electric resistivity of 400 μΩcm andno aging effect under exposure to air. The film density is higher thanthat of Ta—N films formed by typical ALD, in which NH₃ is used insteadof hydrogen radicals. In addition, the as-deposited films are notamorphous, but rather polycrystalline structure of cubit TaN. Thedensity and crystallinity of the films increases with the pulse time ofhydrogen plasma. The films are Ta-rich in composition and contain around15 atomic % of carbon impurity. In the PEALD of Ta—N films, hydrogenradicals are used a reducing agent instead of NH₃, which is used as areactant gas in typical Ta—N ALD. Films are deposited on SiO₂ (100nm)/Si wafers at a deposition temperature of 260° C. and a depositionpressure of 133 Pa in a cold-walled reactor using (Net₂)₃Ta=Nbu^(t)[tertbutylimidotris(diethylamido)tantalum, TBTDET] as aprecursor of Ta. The liquid precursor is contained in a bubbler heatedat 70° C. and carried by 35 sccm argon. One deposition cycle consist ofan exposure to a metallorganic precursor of TBTDET, a purge period withAr, and an exposure to hydrogen plasma, followed by another purge periodwith Ar. The Ar purge period of 15 seconds instead between each reactantgas pulse isolates the reactant gases from each other. To ignite andmaintain the hydrogen plasma synchronized with the deposition cycle, arectangular shaped electrical power is applied between the upper andlower electrode. The showerhead for uniform distribution of the reactantgases in the reactor, capacitively coupled with an rf (13.56 MHz) plasmasource operated at a power of 100 W, is used as the upper electrode. Thelower electrode, on which a wafer resides, is grounded. Film thicknessand morphology were analyzed by field emission scanning electronmicroscopy.

Ta(Al)N(C): Technical work on thin films have been studied using TaCl₅or TaBr₅ and NH₃ as precursors and Al(CH₃)₃ as an additional reducingagent. The deposition temperature is varied between 250 and 400° C. Thefilms contained aluminum, carbon, and chlorine impurities. The chlorinecontent decreased drastically as the deposition temperature isincreased. The film deposited at 400° C. contained less than 4 atomic %chlorine and also had the lowest resistivity, 1300 μΩcm. Five differentdeposition processes with the pulsing orders TaCl₅-TMA-NH₃,TMA-TACl₅-NH₃, TaBr₅-NH₃, TaBr₅—Zn—NH₃, and TaBr₅-TMA-NH₃ are used.TaCl₅, TaBr₅, and Zn are evaporated from open boats held inside thereactor. The evaporation temperatures for TaCl₄, TaBr₅, and Zn are 90,140, 380° C., respectively. Ammonia is introduced into the reactorthrough a mass flowmeter, a needle valve, and a solenoid valve. The flowrate is adjusted to 14 sccm during a continuous flow. TMA is kept at aconstant temperature of 16° C. and pulsed through the needle andsolenoid valve. Pulse times are 0.5 s for TaCl₅, TaBr₅, NH₃, and Znwhereas the pulse length of TMA is varied between 0.2 and 0.8 s. Thelength of the purge pulse is always 0.3 s. Nitrogen gas is used for thetransportation of the precursor and as a purging gas. The flow rate ofnitrogen is 400 sccm.

TiN: Atomic layer deposition (ALD) of amorphous TiN films on SiO₂between 170° C. and 210° C. has been achieved by the alternate supply ofreactant sources, Ti[N(C₂H₅CH3)2]4 [tetrakis(ethylmethylamino)titanium:TEMAT] and NH₃. These reactant sources are injected into the reactor inthe following order: TEMAT vapor pulse, Ar gas pulse, NH3 gas pulse andAr gas pulse. Film thickness per cycle saturated at around 1.6monolayers per cycle with sufficient pulse times of reactant sources at200° C. The results suggest that film thickness per cycle could exceed 1ML/cycle in ALD, and are explained by the rechemisorption mechanism ofthe reactant sources. An ideal linear relationship between number ofcycles and film thickness is confirmed.

TiAlN: Koo et al published paper on the study of the characteristics ofTiAlN thin film deposited by atomic layer deposition method. The seriesof metal-Si—N barriers have high resistivity above 1000 μΩcm. Theyproposed another ternary diffusion barrier of TiAlN. TiAlN filmexhibited a NaCl structure in spite of considerable Al contents. TiAlNfilms are deposited using the TiCl₄ and dimethylaluminum hydrideethypiperdine (DMAH-EPP) as the titanium and aluminum precursors,respectively. TiCl₄ is vaporized from the liquid at 13-15° C. andintroduced into the ALD chamber, which is supplied by a bubbler usingthe Ar carrier gas with a flow rate of 30 sccm. The DMAH-EPP precursoris evaporated at 60° C. and introduced into the ALD chamber with thesame flow rate of TiCl₄. The NH3 gas is also used as a reactant gas andits flow rate is about 60 sccm. Ar purging gas is introduced for thecomplete separation of the source and reactant gases. TiAlN films aredeposited at the temperatures between 350 and 400° C. and total pressureis kept constant to be two torr.

TiSiN: Metal-organic atomic-layer deposition (MOALD) achievesnear-perfect step coverage step and control precisely the thickness andcomposition of grown thin films. A MOALD technique for ternary Ti—Si—Nfilms using a sequential supply of Ti[N(CH₃)₂]₄ [tetrakis(dimethylamido) titanium: TDMAT], silane (SiH₄), and ammonia (NH₃), hasbeen developed and evaluated the Cu diffusion barrier characteristics ofa 10 nm Ti—Si—N film with high-frequency C-V measurements. At 180° C.deposition temperature, silane is supplied separately in the sequence ofthe TDMAT pulse, silane pulse, and the ammonia pulse. The siliconcontent is the deposited films and the deposition thickness per cycleremained almost constant at 18 at. % and 0.22 nm/cycle, even though thesilane partial pressure varied from 0.27 to 13.3 Pa. Especially, the Sicontent dependence is sirikingly different from the conventionalchemical-vapor deposition. Step coverage is approximately 100% even onthe 0.3 μm diameter hole with slightly negative slope and 10:1 aspectratio.

BN: Boron nitride has for the first time been deposited from gaseousBBr₃ and NH₃ by means of atomic layer deposition. The films deposited at750° C. and total pressure of 10 torr on silica substrates showed aturbostratic with a c-axis at 0.7 nm. The film deposited at 400° C. aresignificantly less ordered. The film density is obtained by means ofX-ray reflectivity, and it is found to be 1.65-1.7 and 1.9-1.95 g cm⁻³for the films deposited at 400 and 750° C., respectively. Furthermore,the films are, regardless of deposition temperature, fully transparentand very smooth. The surface roughness is 0.2-0.5 nm as measured byoptical interferometry.

Silicon Nitride: Very recently extremely thin silicon nitride high-k(k=7.2) gate dielectrics have been formed at low temperature (<550° C.)by an atomic-layer-deposition technique with subsequent NH₃ annealing at550° C. A remarkable reduction in leakage current, especially in the lowdielectric voltage region, which will be operating voltage for futuretechnologies, has made it a highly potential gate dielectric for futureultralarge-scale integrated devices. Suppressed soft breakdown eventsare observed in ramped voltage stressing. This suppression is thought tobe due to a strengthened structure of S—N bonds and the smoothness anduniformity at the poly-Si/ALD-silicon-nitride interface. The wafers arecleaned with a NH₄OH:H₂O₂:H₂O=0.15:3:7 solution at 80° C. for 10 min andterminated with hydrogen in 0.5% HF solution to suppress the nativeoxidation. The silicon-nitride gate dielectrics are deposited byalternately supplying SiCl₄ and NH₃ gases. The SiCl₄ exposure at340-375° C. followed by NH₃ exposure at 550° C. is cyclically repeated20 times. The gas pressure of SiCl₄ and NH₃ during the deposition is 170and 300 Torr, respectively. Just after the ALD, NIH₃ annealing iscarried out for 90 min at 550° C. The T_(eq) value of the ALDsilicon-nitride is determined to be 1.2+0.2 nm from the ratio of theaccumulation capacitances of the silicon nitride and the SiO₂ samples.

Silicon-Nitride/SiO₂: An extremely-thin (0.3-0.4 nm) silicon nitridelayer has been deposited on thermally grown SiO₂ by anatomic-layer-deposition technique. The boron penetration through thestack gate dielectric has been dramatically suppressed and thereliability has been significantly improved. An exciting feature of nosoft breakdown (SBD) events is observed in ramped voltage stressing andtime-dependent dielectric breakdown (TDDB) characteristics. After thethermal growth of 2.0 to 3.0 nm thick gate oxides on a Si (001)substrates, silicon nitride layer is deposited by alternately supplyingSiCl₄ and NH₃ gases. The SiCl₄ exposure at 375° C. followed by NH₃exposure at 550° C. is cyclically repeated five times, leading to asilicon nitride thickness of 0.3-0.4 nm. The thickness of the ALDsilicon nitride is confirmed to be controlled with an atomic layer levelby the number of the deposition cycle.

WN: Tungsten nitride films have been deposited with the atomic layercontrol using sequential surface reactions. The tungsten nitride filmgrowth is accomplished by separating the binary reaction2WF₆+NH₃->W₂N+3HF+9/2 F₂ into two half-reactions. Successive applicationof the WF₆ and NH₃ half-reactions in an ABAB . . . sequence producedtungsten nitride deposition at substrate temperatures between 600 and800 K. Transmission Fourier transform infrared (FTIR) spectroscopymonitored the coverage of WF_(x) ⁺ and NH_(y) ⁺ surface species on highsurface area particles during the WF₆ and NH₃ half-reactions. The FTIRspectroscope results demonstrated the WF₆ and NH₃ half-reactions arecomplete and self-limiting at temperatures>600 K. In situ spectroscopicellipsometry monitored the film growth on Si(100) substrate vs.temperature and reactant exposure. A tungsten nitride deposition rate of2.55 Å/AB cycle is measured at 600-800 K for WF₆ and NH₃ reactantexposure>3000 L and 10,000 L, respectively. X-ray photoelectronspectroscopy depth-profiling experiments determined that the films had aW₂N stoichiometry with low C and O impurity concentrations. X-raydiffraction investigations revealed that the tungsten nitride films aremicrocrystalline. Atomic force microscopy measurements of the depositedfilms observed remarkably flat surface indicating smooth film growth.These smooth tungsten nitride films deposited with atomic layer controlshould be used as diffusion control for Cu on contact and via holes.

AlN: Aluminum nitride (AlN) has been grown on porous silica by atomiclayer chemical vapor deposition (ALCVD) from trimethylaluminum (TMA) andammonia precursors. The ALCVD growth is based on alternating, separated,saturating reactions of the gaseous precursors with the solidsubstrates. TMA and ammonia are reacted at 423 and 623 Kelvin (K),respectively, on silica which has been dehydroxylated at 1023 Kpretreated with ammonia at 823 K. The growth in three reaction cycles isinvestigated quntitatively by elemental analysis, and the surfacereaction products are identified by IR and solid state and Si NMRmeasurements. Steady growth of about 2 aluminum atoms/nm²_(silica A)/reaction cycle is obtained. The growth mainly took placethrough (I) the reaction of TMA which resulted in surface Al-Me andSi-Me groups, and (II) the reaction of ammonia which replacedaluminium-bonded methyl groups with amino groups. Ammonia also reactedin part with the silicon-bonded methyl groups formed in the dissociatedreaction of TMA with siloxane bridges. TMA reacted with the aminogroups, as it did with surface silanol groups and siloxane bridges. Ingeneral, the Al—N layer interacted strongly with the silica substrates,but in the third reaction cycle AlN-type sites may have formed.

GaN: Pseudo substrates of GaN templates have been grown by MOCVD onsapphire, apart from the quantum dot samples, which are grown on bulk6H—SiC. Prior to GaN ALE, about 400-nm-thick fully relaxed AlN layersare deposited on all substrates. The N₂ flux has been fixed to 0.5 sccmand the rf power to 300 W, which leads to maximum AlN and GaN growthrates of about 270 nm/h under N-limited metal-rich conditions. The Gaflux has been calibrated by measuring the GaN growth rate under N-richconditions using reflection high-energy electron diffraction (RHEED)oscillations at T_(s)=650° C., where it is safe to assume that the Gasticking coefficient is unity.

Memory Devices

According to the teachings of the present invention, the gate insulatorstructure shown in FIG. 5 is employed in a wide variety of differentflash memory type devices. That is, in embodiments of the presentinvention, the gate structure embodiment of FIG. 5, having siliconoxide-metal oxide-silicon oxide-nitride nanolaminates, is used in placeof the gate structure provided in the following commonly assignedpatents: U.S. Pat. Nos. 5,936,274; 6,143,636; 5,973,356; 6,238,976;5,991,225; 6,153,468; and 6,124,729.

In embodiments of the present invention, the gate structure embodimentof FIG. 5, having silicon oxide-metal oxide-silicon oxide-nitridenanolaminates, is used in place of the gate structure provided in thefollowing commonly assigned pending applications: Forbes, L., “WriteOnce Read Only Memory Employing Charge Trapping In Gate Insulators,”application Ser. No. 10/177,077; Forbes, L., “Write Once Read OnlyMemory Employing Floating Gates,” application Ser. No. 10/177,083;Forbes, L., “Write Once Read Only Memory with Large Work FunchtionFloating Gates,” application Ser. No. 10/177,213; Forbes, L.,“Nanoncrystal Write Once Read Only Memory for Archival Storage,”application Ser. No. 10/177,214; Forbes, L., “Ferroelectric Write OnceRead Only Memory for Archival Storage,” application Ser. No. 10/177,082;Forbes, L., “Vertical NROM Having a Storage Density of 1 Bit Per 1F²,”application Ser. No. 10/177,208; Forbes, L., “Multistate NROM Having aStorage Density Much Greater than 1 Bit Per 1F²,” application Ser. No.10/177,211; Forbes, L., “NOR Flash Memory Cell with High StorageDensity,” application Ser. No. 10/177,483.

According to the teachings of the present invention, embodiments of thenovel transistor herein, which are substituted for the gate structuresdescribed in the references above, are programmed by grounding a sourceline and applying a gate voltage and a voltage to the drain to causechannel hot electron injection. To read the memory state, the drain andground or source have the normal connections and the conductivity of thetransistor determined using low voltages so as not to disturb the memorystate. The devices can be erased by applying a large negative voltage tothe gate.

In embodiments of the present invention, the gate structure embodimentof FIG. 5, having silicon oxide-metal oxide-silicon oxide-nitridenanolaminates, is used in place of the gate structure provided in thefollowing commonly assigned patents: U.S. Pat. Nos. 5,936,274,6,143,636, 5,973,356 and 6,238,976 (vertical flash memory devices withhigh density); 5,991,225 and 6,153,468 (programmable memory address anddecode circuits); and 6,124,729 (programmable logic arrays).

Further, in embodiments of the present invention, the gate structureembodiment of FIG. 5, having silicon oxide-metal oxide-siliconoxide-nitride nanolaminates, is used in place of the gate structureprovided in the following Eitan, B. et al., “NROM: A novel localizedTrapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Lett.,21(11), 543-545 (November 2000); Eitan, B. et al., “Characterization ofChannel Hot Electron Injection by the Subthreshold Slope of NROM device,IEEE Electron Device Lett., 22(11), 556-558 (November 2001); Maayan, E.et al., “A 512 Mb NROM Flash Data Storage Memory with 8 MB/s Data Rate,”Dig. IEEE Int. Solid-State Circuits Conf., 100-101 (2002). In theseembodiments, the gate structure embodiment of FIG. 5, having siliconoxide-metal oxide-silicon oxide-nitride nanolaminates used in place ofthe gate structures in those references, can be programmed in thereverse direction and read in the forward direction to obtain moresensitivity in the device characteristics to the stored charge.

All of the above references are incorporated herein in full. The gatestructure embodiment of FIG. 5, having silicon oxide-siliconoxide-nitride nanolaminates-silicon oxide, are herein used in place ofthe gate structure provided in those references to support the variousembodiments of the present invention. That is, the present inventionincorporates the multitude of device structures described in thosereferences to create a multitude of new embodiments which utilizeelectron trapping in the insulator nanolaminate gate structure shown inFIG. 5, rather than employing floating gates, as recited in many of theabove references.

Sample Operation

FIGS. 7A-B and 8 are embodiments useful in illustrating the use ofcharge storage in the oxide-nitride nanolaminate layers to modulate theconductivity of the transistor cell according to the teachings of thepresent invention. That is, FIGS. 7A-7B illustrates the operation of anembodiment for a novel transistor cell 701 formed according to theteachings of the present invention. And, FIG. 8 illustrates theoperation of a conventional DRAM cell 701. As shown in FIG. 7A, theembodiment consists of a gate insulator stack having insulator layers,710, 708 and 718, e.g. 1. SiO₂/oxide-nitride nanolaminate layers/SiO₂.In the embodiment of FIG. 7A, the gate insulator stack having insulatorlayers, 710, 708 and 718, has a thickness 711 thicker than in aconventional DRAM cell, e.g. 801 and is equal to or greater than 10 nmor 100 Å (10⁻⁶ cm). In the embodiment shown in FIG. 7A a transistor cellhas dimensions 713 of 0.1 μm (10⁻⁵ cm) by 0.1 μm. The capacitance, Ci,of the structure depends on the dielectric constant, ε_(i), and thethickness of the insulating layers, t. In the embodiment, the dielectricconstant is 0.3×10⁻¹² F/cm and the thickness of the insulating layer is10⁻⁶ cm such that Ci=εi/t, Farads/cm² or 3×10⁻⁷ F/cm². In oneembodiment, a charge of 10¹² electrons/cm² is programmed into theoxide-nitride nanolaminate layers of the transistor cell. Here thecharge carriers become trapped in potential wells in the oxide-nitridenanolaminate layers 708 formed by the different electron affinities ofthe insulators 710, 708 and 718, as shown in FIG. 7A. This produces astored charge ΔQ=10¹² electrons/cm²×1.6×10⁻¹⁹ Coulombs. In thisembodiment, the resulting change in the threshold voltage (ΔVt) of thetransistor cell will be approximately 0.5 Volts (ΔVt=ΔQ/Ci or1.6×10⁻⁷/3×10⁻⁷=½ Volt). For ΔQ=10¹² electrons/cm³ in an area of 10⁻¹⁰cm², this embodiment of the present invention involves trapping a chargeof approximately 100 electrons in the oxide-nitride nanolaminate layers708 of the transistor cell. In this embodiment, an original V_(T) isapproximately ½ Volt and the V_(T) with charge trapping is approximately1 Volt.

FIG. 7B aids to further illustrate the conduction behavior of the noveltransistor cell of the present invention. As one of ordinary skill inthe art will understand upon reading this disclosure, if the transistorcell is being driven with a control gate voltage of 1.0 Volt (V) and thenominal threshold voltage without the oxide-nitride nanolaminate layerscharged is ½ V, then if the oxide-nitride nanolaminate layers arecharged the transistor cell of the present invention will be off and notconduct. That is, by trapping a charge of approximately 100 electrons inthe oxide-nitride nanolaminate layers of the transistor cell, havingdimensions of 0.1 μm (10⁻⁵ cm) by 0.1 μm, will raise the thresholdvoltage of the transistor cell to 1.0 Volt and a 1.0 Volt control gatepotential will not be sufficient to turn the device on, e.g. Vt=1.0V,I=0.

Conversely, if the nominal threshold voltage without the oxide-nitridenanolaminate layers charged is ½ V, then I=μC_(ox)×(W/L)×((Vgs−Vt)²/2),or 12.5 μA, with μC_(ox)=μC_(i)=100 μA/V² and W/L=1. That is, thetransistor cell of the present invention, having the dimensions describeabove will produce a current I=100 μA/V²×(¼)×(½)=12.5 μA. Thus, in thepresent invention an un-written, or un-programmed transistor cell canconduct a current of the order 12.5 μA, whereas if the oxide-nitridenanolaminate layers are charged then the transistor cell will notconduct. As one of ordinary skill in the art will understand uponreading this disclosure, the sense amplifiers used in DRAM arrays, andas describe above, can easily detect such differences in current on thebit lines.

By way of comparison, in a conventional DRAM with 30 femtoFarad (fF)storage capacitor 851 charged to 50 femto Columbs (fC), if these areread over 5 nS then the average current on a bit line 852 is only 10 μA(I=50 fC/5 ns=10 μA). Thus, storing a 50 fC charge on the storagecapacitor equates to storing 300,000 electrons (Q=50f/C/(1.6×10⁻¹⁹)=30×10⁴=300.000 electrons).

According to the teachings of the present invention, the transistorcells, having the gate structure with oxide-nitride nanolaminate layers,in the array are utilized not just as passive on or off switches astransfer devices in DRAM arrays but rather as active devices providinggain. In the present invention, to program the transistor cell “off,”requires only a stored charge in the oxide-nitride nanolaminate layersof about 100 electrons if the area is 0.1 μm by 0.1 μm. And, if thetransistor cell is un-programmed, e.g. no stored charge trapped in theoxide-nitride nanolaminate layers, and if the transistor cell isaddressed over 10 nS a current of 12.5 μA is provided. The integrateddrain current then has a charge of 125 fC or 800,000 electrons. This isin comparison to the charge on a DRAM capacitor of 50 fC which is onlyabout 300,000 electrons. Hence, the use of transistor cells, having thegate structure with oxide-nitride nanolaminate layers, in the array asactive devices with gain, rather than just switches, provides anamplification of the stored charge, in the oxide-nitride nanolaminatelayers, from 100 to 800,000 electrons over a read address period of 10nS.

Sample Device Applications

In FIG. 9 a memory device is illustrated according to the teachings ofthe present invention. The memory device 940 contains a memory array942, row and column decoders 944, 948 and a sense amplifier circuit 946.The memory array 942 consists of a plurality of transistor cells 900,having oxide-nitride nanolaminate layers in the gate stack, whose wordlines 980 and bit lines 960 are commonly arranged into rows and columns,respectively. The bit lines 960 of the memory array 942 are connected tothe sense amplifier circuit 946, while its word lines 980 are connectedto the row decoder 944. Address and control signals are input onaddress/control lines 961 into the memory device 940 and connected tothe column decoder 948, sense amplifier circuit 946 and row decoder 944and are used to gain read and write access, among other things, to thememory array 942.

The column decoder 948 is connected to the sense amplifier circuit 946via control and column select signals on column select lines 962. Thesense amplifier circuit 946 receives input data destined for the memoryarray 942 and outputs data read from the memory array 942 overinput/output (I/O) data lines 963. Data is read from the cells of thememory array 942 by activating a word line 980 (via the row decoder944), which couples all of the memory cells corresponding to that wordline to respective bit lines 960, which define the columns of the array.One or more bit lines 960 are also activated. When a particular wordline 980 and bit lines 960 are activated, the sense amplifier circuit946 connected to a bit line column detects and amplifies the conductionsensed through a given transistor cell and transferred to its bit line960 by measuring the potential difference between the activated bit line960 and a reference line which may be an inactive bit line. Again, inthe read operation the source region of a given cell is couple to agrounded sourceline or array plate (not shown). The operation of Memorydevice sense amplifiers is described, for example, in U.S. Pat. Nos.5,627,785; 5,280,205; and 5,042,011, all assigned to Micron TechnologyInc., and incorporated by reference herein.

FIG. 10 shows a conventional NOR-NOR logic array 1000 which isprogrammable at the gate mask level by either fabricating a thin oxidegate transistor, e.g. logic cells 1001-1, 1001-2, . . . , 1001-N and1003-1, 1003-2, . . . , 1003-N, at the intersection of lines in thearray or not fabricating a thin oxide gate transistor, e.g. missing thinoxide transistors, 1002-1, 1002-2, . . . , 1002-N, at such anintersection. As one of ordinary skill in the art will understand uponreading this disclosure, the same technique is conventionally used toform other types of logic arrays not shown. As shown in FIG. 10, anumber of depletion mode NMOS transistors, 1016 and 1018 respectively,are used as load devices.

The conventional logic array shown in FIG. 10 includes a first logicplane 1010 which receives a number of input signals at input lines 1012.In this example, no inverters are provided for generating complements ofthe input signals. However, first logic plane 1010 can include invertersto produce the complementary signals when needed in a specificapplication.

First logic plane 1010 includes a number of thin oxide gate transistors,e.g. transistors 1001-1, 1001-2, . . . , 1001-N. The thin oxide gatetransistors, 1001-1, 1001-2, . . . , 1001-N, are located at theintersection of input lines 1012, and interconnect lines 1014. In theconventional PLA of FIG. 10, this selective fabrication of thin oxidegate transistor, e.g. transistors 1001-1, 1001-2, . . . , 1001-N, isreferred to as programming since the logical function implemented by theprogrammable logic array is entered into the array by the selectivearrangement of the thin oxide gate transistors, or logic cells, 1001-1,1001-2, . . . , 1001-N, at the intersections of input lines 1012, andinterconnect lines 1014 in the array.

In this embodiment, each of the interconnect lines 1014 acts as a NORgate for the input lines 1012 that are connected to the interconnectlines 1014 through the thin oxide gate transistors, 1001-1, 1001-2, . .. , 1001-N, of the array. For example, interconnection line 1014A actsas a NOR gate for the signals on input lines 1012A and 1012B. That is,interconnect line 1014A is maintained at a high potential unless one ormore of the thin oxide gate transistors, 1001-1, 1001-2, . . . , 1001-N,that are coupled to interconnect line 1014A are turned on by a highlogic level signal on one of the input lines 1012. When a control gateaddress is activated, through input lines 1012, each thin oxide gatetransistor, e.g. transistors 1001-1, 1001-2, . . . , 1001-N, conductswhich performs the NOR positive logic circuit function, an inversion ofthe OR circuit function results from inversion of data onto theinterconnect lines 1014 through the thin oxide gate transistors, 1001-1,1001-2, . . . , 1001-N, of the array.

As shown in FIG. 10, a second logic plane 1024 is provided whichincludes a number of thin oxide gate transistor, e.g. transistors1003-1, 1003-2, . . . , 1003-N. The thin oxide gate transistors, 1003-1,1003-2, . . . , 1003-N, are located at the intersection of interconnectlines 1014, and output lines 1020. Here again, the logical function ofthe second logic plane 1024 is implemented by the selective arrangementof the thin oxide gate transistors, 1003-1, 1003-2, . . . , 1003-N, atthe intersections of interconnect lines 1014, and output lines 1020 inthe second logic plane 1024. The second logic plane 1024 is alsoconfigured such that the output lines 1020 comprise a logical NORfunction of the signals from the interconnection lines 1014 that arecoupled to particular output lines 1020 through the thin oxide gatetransistors, 1003-1, 1003-2, . . . , 1003-N, of the second logic plane1024. Thus, in FIG. 10, the incoming signals on each line are used todrive the gates of transistors in the NOR logic array as the same isknown by one of ordinary skill in the art and will be understood byreading this disclosure.

FIG. 11 illustrates an embodiment of a novel in-service programmablelogic array (PLA) formed with logic cells having a gate structure withoxide-nitride nanolaminate layers, according to the teachings of thepresent invention. In FIG. 11, PLA 1100 implements an illustrativelogical function using a two level logic approach. Specifically, PLA1100 includes first and second logic planes 1110 and 1122. In thisexample, the logic function is implemented using NOR-NOR logic. As shownin FIG. 11, first and second logic planes 1110 and 1122 each include anarray of, logic cells, having a gate structure with oxide-nitridenanolaminate layers, which serve as driver transistors, 1101-1, 1101-2,. . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N respectively, formedaccording to the teachings of the present invention. The drivertransistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . ,1102-N, have their first source/drain regions coupled to source lines ora conductive source plane. These driver transistors, 1101-1, 1101-2, . .. , 1101-N, and 1102-1, 1102-2, . . . , 1102-N are configured toimplement the logical function of FPLA 1100. The driver transistors,1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N areshown as n-channel transistors. However, the invention is not solimited. Also, as shown in FIG. 11, a number of p-channel metal oxidesemiconductor (PMOS) transistors are provided as load devicetransistors, 1116 and 1124 respectively, having their source regionscoupled to a voltage potential (VDD). These load device transistors,1116 and 1124 respectively, operate in complement tothedrivertransistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1,1102-2, . . . , 1102-N to form load inverters.

It is noted that the configuration of FIG. 11 is provided by way ofexample and not by way of limitation. Specifically, the teachings of thepresent application are not limited to programmable logic arrays in theNOR-NOR approach. Further, the teachings of the present application arenot limited to the specific logical function shown in FIG. 11. Otherlogical functions can be implemented in a programmable logic array, withthe driver transistors, having a gate structure with oxide-nitridenanolaminate layers, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2,. . . ., 1102-N and load device transistors, 1116 and 1124 respectively,of the present invention, using any one of the various two level logicapproaches.

First logic plane 1110 receives a number of input signals at input lines1112. In this example, no inverters are provided for generatingcomplements of the input signals. However, first logic plane 1110 caninclude inverters to produce the complementary signals when needed in aspecific application.

First logic plane 1110 includes a number of driver transistors, having agate structure with oxide-nitride nanolaminate layers, 1101-1, 1101-2, .. . , 1101-N, that form an array. The driver transistors, 1101-1,1101-2, . . . , 1101-N, are located at the intersection of input lines1112, and interconnect lines 1114. Not all of the driver transistors,1101-1, 1101-2, . . . , 1101-N, are operatively conductive in the firstlogic plane. Rather, the driver transistors, 1101-1, 1101-2, . . . ,1101-N, are selectively programmed, as has been described herein, torespond to the input lines 1112 and change the potential of theinterconnect lines 1114 so as to implement a desired logic function.This selective interconnection is referred to as programming since thelogical function implemented by the programmable logic array is enteredinto the array by the driver transistors, 1101-1, 1101-2, . . . ,1101-N, that are used at the intersections of input lines 1112, andinterconnect lines 1114 in the array.

In this embodiment, each of the interconnect lines 1114 acts as a NORgate for the input lines 1112 that are connected to the interconnectlines 1114 through the driver transistors, 1101-1, 1101-2, . . . ,1101-N, of the array 1100. For example, interconnection line 1114A actsas a NOR gate for the signals on input lines 1112A, 1112B and 1112C.Programmability of the driver transistors, 1101-1, 1101-2, . . . ,1101-N is achieved by trapping charge carriers in potential wells in theoxide-nitride nanolaminate layers of the gate stack, as describedherein. When the oxide-nitride nanolaminate layers are charged, thatdriver transistor, 1101-1, 1101-2, . . . , 1101-N will remain in an offstate until it is reprogrammed. Applying and removing a charge to theoxide-nitride nanolaminate layers, is performed by tunneling charge intothe oxide-nitride nanolaminate layers of the driver transistors, 1101-1,1101-2, . . . , 1101-N. A driver transistors, 1101-1, 1101-2, . . . ,1101-N programmed in an off state remains in that state until the chargeis removed from the oxide-nitride nanolaminate layers.

Driver transistors, 1101-1, 1101-2, . . . , 1101-N not having theircorresponding gate structure with oxide-nitride nanolaminate layerscharged operate in either an on state or an off state, wherein inputsignals received by the input lines 1112A, 1112B and 1112C determine theapplicable state. If any of the input lines 1112A, 1112B and 1112C areturned on by input signals received by the input lines 1112A, 1112B and1112C, then a ground is provided to load device transistors 1116. Theload device transistors 1116 are attached to the interconnect lines1114. The load device transistors 1116 provide a low voltage level whenany one of the driver transistors, 1101-1, 1101-2, . . . , 1101-Nconnected to the corresponding interconnect line 1114 is activated. Thisperforms the NOR logic circuit function, an inversion of the OR circuitfunction results from inversion of data onto the interconnect lines 1114through the driver transistors, 1101-1, 1101-2, . . . , 1101-N of thearray 1100. When the driver transistors, 1101-1, 1101-2, . . . , 1101-Nare in an off state, an open is provided to the drain of the load devicetransistors 1116. The VDD voltage level is applied to correspondinginput lines, e.g. the interconnect lines 1114 for second logic plane1122 when a load device transistors 1116 is turned on by a clock signalreceived at the gate of the load device transistors 1116. Each of thedriver transistors, 1101-1, 1101-2, . . . , 1101-N described herein areformed according to the teachings of the present, having a gatestructure with oxide-nitride nanolaminate layers.

In a similar manner, second logic plane 1122 comprises a second array ofdriver transistors, 1102-1, 1102-2, . . . , 1102-N that are selectivelyprogrammed to provide the second level of the two level logic needed toimplement a specific logical function. In this embodiment, the array ofdriver transistors, 1102-1, 1102-2, . . . , 1102-N is also configuredsuch that the output lines 1120 comprise a logical NOR function of thesignals from the interconnection lines 1114 that are coupled toparticular output lines 1120 through the driver transistors, 1102-1,1102-2, . . . , 1102-N of the second logic plane 1122.

Programmability of the driver transistors, 1102-1, 1102-2, . . . ,1102-N is achieved by trapping charge carriers in potential wells in theoxide-nitride nanolaminate layers of the gate stack, as describedherein. When the oxide-nitride nanolaminate layers are charged, thatdriver transistor, 1102-1, 1102-2, . . . , 1102-N will remain in an offstate until it is reprogrammed. Applying and removing a charge to theoxide-nitride nanolaminate layers are performed by tunneling charge intothe oxide-nitride nanolaminate layers of the driver transistors, 1101-1,1101-2, . . . , 1101-N. A driver transistor, e.g. 1102-1, 1102-2, . . ., 1102-N, programmed in an off state remains in that state until thecharge is removed from the oxide-nitride nanolaminate layers.

Driver transistors, 1102-1, 1102-2, . . . , 1102-N not having theircorresponding gate structure with oxide-nitride nanolaminate layerscharged operate in either an on state or an off state, wherein signalsreceived by the interconnect lines 1114 determine the applicable state.If any of the interconnect lines 1114 are turned on, then a ground isprovided to load device transistors 1124 by applying a ground potentialto the source line or conductive source plane coupled to the transistorsfirst source/drain region as described herein. The load devicetransistors 1124 are attached to the output lines 1120. The load devicetransistors 1124 provide a low voltage level when any one of the drivertransistors, 1102-1, 1102-2, . . . , 1102-N connected to thecorresponding output line is activated. This performs the NOR logiccircuit function, an inversion of the OR circuit function results frominversion of data onto the output lines 1120 through the drivertransistors, 1102-1, 1102-2, . . . , 1102-N of the array 1100. When thedriver transistors, 1102-1, 1102-2, . . . , 1102-N are in an off state,an open is provided to the drain of the load device transistors 1124.The VDD voltage level is applied to corresponding output lines 1120 forsecond logic plane 1122 when a load device transistor 1124 is turned onby a clock signal received at the gate of the load device transistors1124. In this manner a NOR-NOR electrically programmable logic array ismost easily implemented utilizing the normal PLA array structure. Eachof the driver transistors, 1102-1, 1102-2, . . . , 1102-N describedherein are formed according to the teachings of the present, having agate structure with oxide-nitride nanolaminate layers.

Thus FIG. 11 shows an embodiment for the application of the noveltransistor cells, having a gate structure with oxide-nitridenanolaminate layers, in a logic array. If a driver transistors, 1101-1,1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N, isprogrammed with a negative charge trapped in potential wells, formedwith the oxide-nitride nanolaminate layers, it is effectively removedfrom the array. In this manner the array logic functions can beprogrammed even when the circuit is in the final circuit or in the fieldand being used in a system.

The absence or presence of charge trapped in potential wells, formed bythe oxide-nitride nanolaminate layers, is read by addressing the inputlines 1112 or control gate lines and y-column/sourcelines to form acoincidence in address at a particular logic cell. The control gate linewould for instance be driven positive at some voltage of 1.0 Volts andthe y-column/sourceline grounded, if the oxide-nitride nanolaminatelayers are not charged with electrons then the transistor would turn ontending to hold the interconnect line on that particular row downindicating the presence of a stored “one” in the cell. If thisparticular transistor cell has charge trapped in potential wells, formedby the oxide-nitride nanolaminate layers, the transistor will not turnon and the presence of a stored “zero” is indicated in the cell. In thismanner, data stored on a particular transistor cell can be read.

Programming can be achieved by hot electron injection. In this case, theinterconnect lines, coupled to the second source/drain region for thetransistor cells in the first logic plane, are driven with a higherdrain voltage like 2 Volts for 0.1 micron technology and the controlgate line is addressed by some nominal voltage in the range of twicethis value. Erasure is accomplished by driving the control gate linewith a large positive voltage and the sourceline and/or backgate orsubstrate/well address line of the transistor with a negative bias sothe total voltage difference is in the order of 3 Volts causingelectrons to tunnel out of the oxide-nitride nanolaminate layers of thedriver transistors. Writing can be performed, as also described above,by normal channel hot electron injection

One of ordinary skill in the art will appreciate upon reading thisdisclosure that a number of different configurations for the spatialrelationship, or orientation of the input lines 1112, interconnect lines1114, and output lines 1120 are possible.

FIG. 12 is a block diagram of an electrical system, or processor-basedsystem, 1200 utilizing transistor cells with a gate structure havingoxide-nitride nanolaminate layers. By way of example and not by way oflimitation, memory 1212 is constructed in accordance with the presentinvention to have transistor cells with a gate structure havingoxide-nitride nanolaminate layers. The same applies to transistors inthe CPU, etc., the invention is not so limited. The processor-basedsystem 1200 may be a computer system, a process control system or anyother system employing a processor and associated memory. The system1200 includes a central processing unit (CPU) 1202, e.g., amicroprocessor, that communicates with the NOR flash memory 1212 and anI/O device 1208 over a bus 1220. It must be noted that the bus 1220 maybe a series of buses and bridges commonly used in a processor-basedsystem, but for convenience purposes only, the bus 1220 has beenillustrated as a single bus. A second I/O device 1210 is illustrated,but is not necessary to practice the invention. The processor-basedsystem 1200 can also includes read-only memory (ROM) 1214 and mayinclude peripheral devices such as a floppy disk drive 1204 and acompact disk (CD) ROM drive 1206 that also communicates with the CPU1202 over the bus 1220 as is well known in the art.

It will be appreciated by those skilled in the art that additionalcircuitry and control signals can be provided, and that the memorydevice 1200 has been simplified to help focus on the invention. In oneembodiment, at least one of the transistor cells, having a gatestructure with oxide-nitride nanolaminate layers in memory 1212 includesa programmed transistor cell according to the teachings of the presentinvention.

It will be understood that the embodiment shown in FIG. 12 illustratesan embodiment for electronic system circuitry in which the noveltransistor cells of the present invention are used. The illustration ofsystem 1200, as shown in FIG. 12, is intended to provide a generalunderstanding of one application for the structure and circuitry of thepresent invention, and is not intended to serve as a completedescription of all the elements and features of an electronic systemusing the novel transistor cell structures. Further, the invention isequally applicable to any size and type of memory device 1200 using thenovel transistor cells of the present invention and is not intended tobe limited to that described above. As one of ordinary skill in the artwill understand, such an electronic system can be fabricated insingle-package processing units, or even on a single semiconductor chip,in order to reduce the communication time between the processor and thememory device.

Applications containing the novel transistor cell of the presentinvention as described in this disclosure include electronic systems foruse in memory modules, device drivers, power modules, communicationmodems, processor modules, and application-specific modules, and mayinclude multilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

CONCLUSION

This disclosure describes the use of oxide-nitride nanolaminate layerswith charge trapping in potential wells formed by the different electronaffinities of the insulator layers. The disclosure describes thefabrication by atomic layer deposition, ALD, and use ofoxide-nitride-oxide nanolaminates. In embodiments of the invention,these nitride material are of the order of 4 nanometers in thickness,with a range of 1 to 10 nm. The compositions of these materials areadjusted so as that they have an electron affinity less than siliconoxide which is 4.1 eV, resulting in a positive conduction band offset.The gate insulator structure embodiments of the present invention,having silicon oxide-metal oxide-silicon oxide-nitride nanolaminates,are employed in a wide variety of different device applications.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A method for operating a memory, comprising: writing to one or morevertical MOSFETs arranged in rows and columns extending outwardly from asubstrate and separated by trenches in a DRAM array in a reversedirection, wherein each MOSFET in the DRAM array includes a sourceregion, a drain region, a channel region between the source and thedrain regions, and a gate separated from the channel region by a gateinsulator in the trenches, wherein the gate insulator includesoxide-nitride nanolaminate layers with charge trapping in potentialwells formed by different electron affinities of the oxide-nitridenanolaminate layers, wherein the DRAM array includes a number ofsourcelines formed in a bottom of the trenches between rows of thevertical MOSFETs and coupled to the source regions of each transistoralong rows the vertical MOSFETs, wherein along columns of the verticalMOSFETs the source region of each column adjacent vertical MOSFET coupleto the sourceline in a shared trench, and wherein the DRAM arrayincludes a number of bitlines coupled to the drain region along rows inthe DRAM array, and wherein programming the one or more vertical MOSFETsin the reverse direction includes; biasing a sourceline for two columnadjacent vertical MOSFETs sharing a trench to a voltage higher than VDD;grounding a bitline coupled to one of the drain regions of the twocolumn adjacent vertical MOSFETs in the vertical MOSFET to beprogrammed; applying a gate potential to the gate for each of the twocolumn adjacent vertical MOSFETs to create a hot electron injection intothe gate insulator of the vertical MOSFET to be programmed adjacent tothe source region such that an addressed MOSFET becomes a programmedMOSFET and will operate at reduced drain source current in a forwarddirection; reading one or more vertical MOSFETs in the DRAM array in aforward direction, wherein reading the one or more MOSFETs in theforward direction includes; grounding a sourceline for two columnadjacent vertical MOSFETs sharing a trench; precharging the drainregions of the two column adjacent vertical MOSFETs sharing a trench toa fractional voltage of VDD; and applying a gate potential ofapproximately 1.0 Volt to the gate for each of the two column adjacentvertical MOSFETs sharing a trench such that a conductivity state of anaddressed vertical MOSFET can be compared to a conductivity state of areference cell.
 2. The method of claim 1, wherein creating a hotelectron injection into the gate insulator of the addressed MOSFETadjacent to the source region includes creating a first thresholdvoltage region (Vt1) adjacent to the drain region and creating a secondthreshold voltage region (Vt2) adjacent to the source region, whereinVt2 is greater that Vt1.
 3. The method of claim 1, wherein reading oneor more vertical MOSFETs in the DRAM array in a forward directionincludes using a sense amplifier to detect whether an addressed verticalMOSFET is a programmed vertical MOSFET, wherein a programmed verticalMOSFET will not conduct, and wherein an un-programmed vertical MOSFETaddressed over approximately 10 ns will conduct a current ofapproximately 12.5 μA such that the method includes detecting anintegrated drain current having a charge of 800,000 electrons using thesense amplifier.
 4. The method of claim 1, wherein in creating a hotelectron injection into the gate insulator of an addressed verticalMOSFET includes changing a threshold voltage for the vertical MOSFET byapproximately 0.5 Volts.
 5. The method of claim 1, wherein creating ahot electron injection into the gate insulator of the addressed verticalMOSFET includes trapping a stored charge in the gate insulator of theaddressed vertical MOSFET of approximately 10¹² electrons/cm².
 6. Themethod of claim 1, wherein creating a hot electron injection into thegate insulator of the addressed vertical MOSFET includes trapping astored charge in the gate insulator of the addressed vertical MOSFET ofapproximately 100 electrons.
 7. The method of claim 1, wherein themethod further includes using the vertical MOSFET as active device withgain, and wherein reading a programmed vertical MOSFET includesproviding an amplification of a stored charge in the gate insulator from100 to 800,000 electrons over a read address period of approximately 10ns.
 8. A method of operating a memory array, comprising: writing amemory state to at least one of a plurality of vertical transistors inan array, including; storing a charge within a storage layer of ananolaminate insulator, wherein the storage layer is formed using atomiclayer deposition techniques, and the nanolaminate insulator is locatedon a trench wall, and wherein the nanolaminate insulator layer separatesa channel region within the trench wall from a gate located over thenanolaminate insulator within the trench; wherein storing the charge isperformed in a first direction; and reading the memory state of thevertical transistor in a second direction opposite from the firstdirection.
 9. The method of claim 8, further including comparing asource/drain current in the vertical transistor a reference source/draincurrent in a second vertical transistor.
 10. The method of claim 9,wherein comparing the source/drain current to the reference source/draincurrent includes comparing to a reference source/drain current in asecond vertical transistor located on an adjacent wall of the trench.11. The method of claim 8, wherein storing the charge within the storagelayer of a nanolaminate insulator includes storing a charge within astorage layer less than or equal to 10 nanometers thick.
 12. The methodof claim 1, wherein storing the charge within the storage layer lessthan or equal to 10 nanometers thick includes storing a charge within astorage layer approximately 4 nanometers thick.
 13. A method ofoperating a memory array, comprising: writing a memory state to at leastone of a plurality of vertical transistors in an array, including;storing a charge within a storage layer of a nanolaminate insulator,wherein the storage layer is formed using atomic layer depositiontechniques, and the nanolaminate insulator is located on a trench wall,and wherein the nanolaminate insulator layer separates a channel regionwithin the trench wall from a gate located over the nanolaminateinsulator within the trench; wherein storing the charge is performed ina first direction; and reading the memory state of the verticaltransistor in a second direction opposite from the first direction,including comparing a source/drain current of the vertical transistor inthe second direction with source/drain current of a vertical referencetransistor on an adjacent side of the trench.
 14. The method of claim13, wherein comparing a source/drain current of the vertical transistorin the second direction with source/drain current of a verticalreference transistor includes utilizing a common source line buried in abase of the trench.
 15. The method of claim 13, wherein comparing asource/drain current of the vertical transistor in the second directionwith source/drain current of a vertical reference transistor includesutilizing a gate that is shared between the vertical transistor and thereference vertical transistor.
 16. The method of claim 13, whereinstoring the charge within the storage layer of a nanolaminate insulatorincludes storing a charge within a storage layer less than or equal to10 nanometers thick.
 17. The method of claim 16, wherein storing thecharge within the storage layer less than or equal to 10 nanometersthick includes storing a charge within a storage layer approximately 4nanometers thick.
 18. A method of operating a memory array, comprising:writing a memory state to at least one of a plurality of verticaltransistors in an array, including; storing a charge within a nitridestorage layer of a nanolaminate insulator, wherein the nitride storagelayer is formed using atomic layer deposition techniques, and thenanolaminate insulator is located on a trench wall, and wherein thenanolaminate insulator layer separates a channel region within thetrench wall from a gate located over the nanolaminate insulator withinthe trench; wherein storing the charge is performed in a firstdirection; and reading the memory state of the vertical transistor in asecond direction opposite from the first direction.
 19. The method ofclaim 18, wherein storing a charge within a nitride storage layerincludes storing a charge within a storage layer with an electronaffinity less than 4.1 eV providing a positive conduction band offsetwith silicon oxide.
 20. The method of claim 18, wherein storing a chargewithin a nitride storage layer includes storing a charge within asilicon nitride storage layer.
 21. The method of claim 18, whereinstoring a charge within a nitride storage layer includes storing acharge within an aluminum nitride storage layer.
 22. The method of claim18, wherein storing a charge within a nitride storage layer includesstoring a charge within a gallium nitride storage layer.
 23. The methodof claim 18, wherein storing a charge within a nitride storage layerincludes storing a charge within a gallium aluminum nitride storagelayer.
 24. The method of claim 18, wherein storing a charge within anitride storage layer includes storing a charge within a tantalumaluminum nitride storage layer.
 25. The method of claim 18, whereinstoring a charge within a nitride storage layer includes storing acharge within a titanium silicon nitride storage layer.
 26. The methodof claim 18, wherein storing a charge within a nitride storage layerincludes storing a charge within a titanium aluminum nitride storagelayer.
 27. The method of claim 18, wherein storing a charge within anitride storage layer includes storing a charge within a tungstenaluminum nitride storage layer.